Film is pretty much just recap of 1 season of the OverLord with few bonus scenes without the final act.
I hate how this is a recap... I thought it might be some sort of sidestory (like other animes do), but nope. Not worth watching imo just watch the anime instead...
Not very useful as a recap - only covers half a season, and doesn't cut the length down by much.
The only reason to watch this instead of just re-running S1 would be to skip the Intro/Ending a few times, and you'd still only be up to episode 7.
Its just season 1 of overlord in one go with small scenes missing and the last ark...